Tides, the periodic rise and fall of the surface of the waters of the earth, are produced principally by the gravitational forces and relative motion between the earth, the moon and the sun. The lunar tide is produced by differences between the constant centrifugal force which acts upon the earth and the gravitational force which tends to draw the earth and the moon towards one another. The greatest gravitional force of attraction is at that point on the earth's surface nearest the moon, while the weakest force of attraction upon the earth's surface is on the opposite side of the earth's surface. There is thus a high water bulge on the earth's surface on the side toward the moon, and another bulge on the side of the earth away from the moon. There is produced a low-water trough on each side of the earth between the bulges. The half lunar day is 12 hours and 25 minutes, which is the time interval between successive high tides, or low tides. An approximately nineteen year period is required for the regression of the moon's nodes to complete a circuit of 360.degree. in longitude. The sun produces upon the earth a solar tide in similar fashion for similar reason, though the effect of the sun on tides is about 46 percent of that of the moon because, despite its mass it is far more distant from the earth. Tidal analysis, for these reasons requires a consideration of the cumulative effects of both the moon and the sun, as well as a consideration of the location of the earth, the moon and the sun one in relation to the other; which differs at different times of the year. These effects are cyclic, and predictable, and annual tidetables are published by the U.S. Coast and Geodetic Survey for the entire world. Each volume contains daily predictions for key places and tables of time and height differences for secondary sites. There are two annual volumes of tidal current tables, primarily for places in North America. Meteorological tides, or tides produced by earthquakes, variable atmospheric pressure and wind, are not predictable, or cyclic, but their effect is generally far less pronounced than that of lunisolar tides.
Ocean tides are directly affected by the positions, and relative movement of the moon and the sun. Ocean tides also profoundly effect the flow of water into and out of estuaries. In restricted channels too a restricted stream exhibits a regular pattern of ebb and flood. In an estuary the current continues to flow upstream for a considerable time after high water. The current in an estuary runs downstream similarly after low water. A flood current is produced when the current is directed toward land, or up an estuary. Conversely, an ebb current is produced when water runs away from the land, or down an estuary.
The time of high water is directly related to the positions of the moon and sun. The length of time between the moon's crossing of the meridian at a given place on the earth and the next high water at that place is known as the lunitidal interval, or the high water interval for that particular place. Similarly, the length of time between the moon's crossing at that place and the next low water is called the low water interval. The duration of the mean rise and fall of tides for a given coastal site is calculated empirically to provide average conditions over a nineteen year cycle, and this information is published in tidetables. Tides are constituted of semi-diurnal, diurnal components and mixed components. The sea shores, river estuaries and tide channels exhibit tidal profiles. Even though tides can be of approximately 5 hour flood and 7 hour ebb, the duration of the average interval between successive high water and low water of semi-diurnal tides is normally 6 hours and 13 minutes, and between successive high waters 12 hours and 25 minutes.
The mechanism of feeding behavior in fishes is complicated and only partially understood. Nonetheless, fishermen have long believed, and have probably observed, that fish generally bite better when the tide is running. Conversely, they believe, and have probably observed, that fish do not generally bite well when there is no current. It is thus believed that the current pushes the small animals on which fish feed, and smaller fish out of their hiding places to excite the curiosity and rapacity of the larger fish; this making the smaller fish more vulnerable to attack by the larger fish. The flow of water is thus associated with a connected series of events which brings about meal time for the fish, the large fish feeding on the smaller fish, and they in turn providing food for yet larger fish. Fish, according to species and prevailing conditions such as the time of year, temperature, and time of day feed just under the surface or at various depths right down to the bottom of the water, but essentially always in relation to the strength of the current. Knowing at a given location the date and time that currents will occur, it is postulated, would be of keen interest to fishermen. Such information should also be of considerable interest to those who navigate such waters. Unfortunately however tidetables issued periodically by the National Ocean Service for specific locations obscure at best the periods of current flow, and do not indicate when currents will be sufficient to bring animals and fish as food from their hiding places to initiate the feeding cycle. Rather, these tables show, over a 24 hour period for any given year, month, and date, for a given location, the predicted hourly heights of the surface of the water on an hourly basis. Such data offers little if anything intelligible to one in need of exacting information, at a given location, regarding the exact time and periods when currents will occur, and the relative strength of current flow.